The invention described herein is generally related to angular orientation sensors. More particularly, this invention relates to remote angular orientation sensors using digital serial encoding. Many applications require an angular orientation position sensor which can be read from a distant location. For example in many robotic systems it is necessary to monitor a tool position. With the advent of fiber optic technology, many remote angular orientation sensing applications are emerging. They include control of industrial processes and remotely piloted vehicles such as submersible marine vehicles.
Angle-sensing, and in particular remote angle sensing, traditionally has been performed primarily by electrical means. One type of sensor, a heading compass, manufactured by Digicourse of New Orleans, La., uses a remotely sensed magnetic compass with a digital code wheel and a ten bit parallel, Grey code digital mask acting as the compass card. A combination of miniature light emitting diodes (LED's) and photo detectors are used to sense the mask position and provide a bit-parallel electrical signal that can be decoded to indicate compass heading. The output of the sensor is an electrical signal that must be carried by wires. This device requires a data link composed of electrical wires that run to and from the sensor. Increasing the length of the data link in this system demands increased power consumption due to Joule heating, and requires a larger power supply.
One type of system that provides remote angle sensing but which does not require electricity at the sensor is described in Lewis, Norris, "Fiber Optic Sensors Offer Advantages For Aircraft," Optical Engineering Reports, June 1987. In this system, the position sensor is a reflective code wheel imprinted with a 10 channel digital mask pattern. "On" and "off" pulses corresponding to code wheel position are superimposed onto carrier light signals that pass through the code wheel to create encoded composite signals. The composite signals propagate through optical fibers to a remotely located position detecting circuit. This system requires expensive diffraction gratings and 10 channels to provide adequate resolution. Each channel requires separate hardware and results in a complex system with many components along with accompanying expense.
Still another remote angle sensing system is described in Varshneya, D., Maida, J. L. and Hakman, E. D., "Fiber Optic Rotary Position Transducer," Fiber Optic and Laser Sensors Vs. P. B. DePaula and E. Udd, ed. Proc. SPIE, 838 (1987). In that device, position sensing is accomplished by transmitting an optical signal via a single optical fiber to a fiber optic read head. Time division multiplexed light signals emitted from the read head interrogate a 10 channel digitally encoded reflective Grey-code encoder disc. Light signals reflect off the disc, reenter the read head, and then are propagated to an optical signal processor. Surface quality and flatness of the absorptive areas of the disc are important factors which affect signal-to-noise ratio and the threshold level of the optical signal processor. Successful operation of this device requires careful alignment of the disc surface and the fiber optic read head. Deviations from normal incidence between the path of the light signals emitted from the read head and the disc severely affect the operational characteristics of this system.
Another remote angle sensor that utilizes encoded optical signals to convey information regarding angular position has been described by Migliori, et al, in U.S. Pat. No. 4,577,414. Migliori teaches a remotely readable compass which operates by comparing the intensities of four light beams, two of which pass through a sheet polarizer. However, the Migliori device requires at least four channels to provide adequate resolution.
All of the remote angle sensing systems taught by the prior art are unduly complex and expensive because they require multiple channels to provide sufficient angular resolution. Each channel increases system complexity and requires additional components. Disadvantageously, those prior art systems which utilize reflective digital discs require critical positioning of the fiber optic read head with respect to the discs. Disc surface tolerances are also critical and increase manufacturing costs. Thus, a need exists for a simple, less expensive, and more easily manufactured remote angle sensing system.